The present disclosure relates generally to a magnetic field generating apparatus for magnetic resonance imaging, and particularly to a magnetic field generating apparatus with compensation of the variation of the central field.
Magnetic resonance imaging (MRI) for medical diagnostics or otherwise requires a magnetic field generating apparatus capable of producing a high degree of final homogeneity in the magnetic field generated in the imaging volume. To accomplish this high degree of homogeneity, shimming systems are used, which often include passive shimming elements. Steel shim elements placed inside the patient bore and saturated by the main field provide a desirable degree of compensation to offset manufacturing and environmental inhomogeneities inherent in the MRI system. The total amount and location of the shimming steel varies from magnet to magnet, where large amounts of steel may be required in magnets with high original inhomogeneity, or in magnets situated in a strong magnetic environment.
With shimming steel, saturation magnetization of the shim elements decreases with temperature, that is, dBsat/dT<0, thereby resulting in a compensation system that is temperature sensitive. Shim elements positioned between the gradient coils and the bore of the magnet vacuum vessel are subject to heating coming from the gradient coils during scanning. Most of the shim elements in the MRI system produce a negative contribution to the central field B0, where B0 is Bz at z=0, that decreases with temperature, such that as the temperature rises, the central field B0 in the magnet rises. Since imaging RF frequency is directly proportional to the central field B0, a stable central field B0 is important for generating a high image quality from the MRI system. The temperature sensitivity of the B0 shimming system imposes limitations on the allowable amount of shimming steel that may be used, thereby effecting the overall shimming capacity.
Acquiring an ability to compensate for B0 temperature sensitivity of the shimming system would release the above restrictions, allow an increase in shimming capacity, and enable different design solutions in an MRI scanner system. Accordingly, there is a need in the art for an improved MRI shimming system.